1. Field of the Invention
The present invention relates to an expansion injection molding die in which the die cavity is pressurized by a gas before expandable thermoplastic resin is injected.
2. Description of the Prior Art
An example of the conventional expansion injection molding die of this kind is shown in FIGS. 1 and 2. In FIG. 1, reference numerals 1 and 2 represent cavity plates; 3, 4 and 5 core plates; 6 a die cavity and liner portion formed between the cavity plates 1, 2 and the core plates 3, 4, 5; 7 a gas introducing passage to supply the pressurized gas into the die cavity 6, and 8 a gas discharge passage to draw the pressurized gas out of the die cavity 6; 9 a backup plate; 10 a spacer block; 11 a large number of ejector pins to strike the molded products out of the die cavity 6; 12 an ejector plate for holding the ejector pins 11; and 13 and 14 upper and lower mounting plates.
The expansion molding using this type of expansion injection molding die consists of the following molding steps. The die cavity 6 is sealed with a gas under pressure before being injected with expandable molten resin material. When the resin is injected, a relief valve is opened to remove the corresponding volume of gas from the die cavity and thereby keep the interior of the die cavity 6 at a constant pressure. This requires a sealing structure with high sealing performance that can keep the pressure in the die cavity 6.
In one of the known conventional sealing structures of this kind, the ejector plate 12 holding the ejector pins 11 is installed in a large hermetically enclosed chamber (Japanese Utility Model Examined Publication No. 4743/1985). Instead of using the large hermetically enclosed chamber, however, the sealing structure generally uses an O-ring 15 which is provided for each moving ejector pin 11 as shown in FIG. 1.
In the latter sealing method, as shown in FIG. 2, a hollow portion 17 larger in cross section than a hole 16--which is cut through the core plates 3, 4, 5 and the backup plate 9 and through which the ejector pin 11 passes--is formed somewhere in the hole 16 in the core plate 5 on the side contacting the backup plate 9. In the hollow portion 17, the gas-sealing O-ring 15 is installed so that it is in sliding contact with the ejector pin 11 for tight sealing. This sealing method has the advantage of not having to seal a large chamber in which the ejector plate 12 is accommodated. Another advantage is that only a small mount of gas is required to be supplied under pressure.
The above ejector pin sealing structure, however, requires the provision of the hollow portions 17 in the core plate in numbers equal to the number of the O-rings 15. In dies where a large number of ejector pins 11 need to be installed (generally a die for molding large-size products has 50 to 150 ejector pins), it is difficult to attain a specified level of centering precision. In this construction, the machining accuracy of the hollow portion 17 directly affects the sealing performance of the O-ring 15. Furthermore, the ejector pin diameter is generally small, about 3 to 10 mm, and the corresponding diameter of the O-ring 15 is also as small as 1.9 mm. And the flattening size of the O-ring 15 is about 0.28 to 0.47 mm. With these small dimensions, it is difficult to attain the specified machining accuracy of the hollow portion 17, which is formed by cutting the core plate 5. Moreover, since the hollow portions 17 are provided in large numbers, the correct relative positioning between the hollow portions 17 and the holes 16 cannot easily be obtained with a desired precision.
As a result, when the ejector pin 11 is inserted into the hole 16 during the assembly of the die, the front end 11a of the ejector pin 11 may damage the O-ring 15, as shown in FIG. 3. This will lead to a sealing failure after some period of operation, making this sealing structure unfit for long period of service.
There is a further disadvantage with this sealing structure. Since the desired centering precision is difficult to obtain, it is necessary to carefully insert the ejector pins 11 one at a time during assembling, put a cap at the front end of each ejector pin 11, guide the O-rings 15, or round the front ends of the ejector pins 11. In addition, the die assembly must be completely overhauled during repair, making the assembly work complex and increasing the number of assembly processes.